How it works
Modbus dates from 1979. A client asks a device for a block of numbered registers, and the device returns raw 16-bit values. Modbus TCP carries this over Ethernet, and Modbus RTU runs over serial lines. Nothing in the message says what a register means. The meaning comes from the manufacturer's register map, which lists each address with its scaling, unit and data type. A value of 4,215 might be 421.5 V if the map gives a scale of 0.1.
OPC UA, maintained by the OPC Foundation, carries structure with the data. A server publishes a tree of named nodes, each with a type, unit and timestamp, and clients can browse it. Clients can subscribe, so they receive changes instead of polling. Encryption, authentication and certificates are built in. Modbus has none of these, so it is kept on isolated networks. Many plants use both, with Modbus at the inverter or meter and OPC UA from the SCADA server outward.
Why it matters
Reading a plant's data starts with these protocols. Each manufacturer's register map is different, and maps can change between firmware versions. Integrating a new plant means mapping hundreds of registers correctly, and one wrong scale factor corrupts every report built on it. In a portfolio with mixed equipment, protocol work is often the slowest step to usable data. Security rules also decide where each protocol is allowed to run.
A worked example
An inverter's map lists active power as a signed 16-bit register with a scale of 0.1 kW. A reading of 27,530 means 27,530 × 0.1 = 2,753 kW, or 2.753 MW. An integrator who applies a scale of 1 shows 27.53 MW on the dashboard, ten times the truth. Polling 60 inverters for 50 registers each, once a second, is 60 × 50 = 3,000 registers a second. One Modbus request can read up to 125 consecutive registers, so with contiguous maps that is 60 requests a second.
Illustrative figures.
Across markets
Inside the plant, Modbus and OPC UA are common everywhere. The link to the grid operator differs. US utilities and system operators commonly use DNP3. European and Indian grid operators commonly use IEC 60870-5-104 for telemetry to their control centres, with IEC 61850 inside substations. A plant built for one market may need a different gateway to report in another.